Method and device for irradiating spots on a layer

ABSTRACT

For irradiating a layer a radiation beam is directed and focussed to a spot on the layer, relative movement of the layer relative to the optical element is caused so that, successively, different portions of the layer are irradiated and an interspace between a surface of the optical element nearest to the layer is maintained. Furthermore, at least a portion of the interspace through which the radiation irradiates the spot on the layer is maintained filled with a liquid, the liquid being supplied via a supply conduit and flowing out of an outflow opening through a total projected cross-sectional passage area in a plane parallel to the layer. The outflow opening or a plurality of the outflow openings are positioned such that, seen in a direction perpendicular to the layer, the total cross-sectional area has a centre in the portion of the interspace through which the radiation irradiates the spot.

The invention relates to a method of irradiating a layer and to a devicefor irradiating a layer.

Such a method and such a device are known from WO-A-02/13194. Accordingto this publication, the described method and device are used for themanufacturing of an optically scannable information carrier. In such aprocess, first a master mold is manufactured, and then, by means of themaster mold or by means of a daughter mold manufactured by means of themaster mold, the information carrier is manufactured by means of areplica process. For manufacturing the master mold, a modulatedradiation beam which is directed and focussed to a scanning spot on aphotosensitive layer carried by a substrate by means of an optical lenssystem and the substrate and the lens system are moved relatively toeach other. An interspace between the photosensitive layer and a nearestsurface of a lens system facing the photosensitive layer is maintainedfilled up with a liquid.

For moving the substrate relative to the lens system a table carryingthe substrate can be rotated about an axis of rotation. By means of adisplacement device, the lens system can be displaced with a radialdirectional component with respect to the axis of rotation of the table.A liquid supply means supplies the liquid into the interspace betweenthe photosensitive layer and a nearest optical surface of the lenssystem.

A problem of this known method and device is that the immersion of thesuccessive portions of the layer to be irradiated is quite easilydisrupted, for instance because the liquid is entrained away from thearea of the interspace through which the radiation directed to theradiation spot passes when the layer and the lens move too quicklyrelative to each other. The immersion can also be disrupted due toimportant changes in the direction of movement of the lens and the layerrelative to each other. The stability of the liquid film between thelayer to be irradiated and the nearest optical surface of the opticalelement can be improved by making the distance between the layer to beirradiated and the nearest optical surface of the optical element verysmall. However, this entails that the device and in particular the lensnearest to the layer to be irradiated can easily be damaged in the eventof contact between the lens and the layer moving relative to each other.

Another method and device for directing a radiation beam to a spot on aphotosensitive layer are disclosed in JP-A-10255319. In accordance withthis method, a photosensitive layer is applied to a disc-shapedsubstrate made from glass. The table and the substrate are rotated aboutan axis of rotation extending perpendicularly to the substrate, and thelens system is displaced, at a comparatively low rate, in a radialdirection with respect to the axis of rotation, so that the scanningspot of the radiation beam formed on the photosensitive layer follows aspiral-shaped track on the photosensitive layer. The radiation beam—inthis known device a laser beam—is modulated such that a series ofirradiated and non-irradiated elements is formed on the spiral-shapedtrack, which series correspond to a desired series of informationelements on the information carrier to be manufactured. Thephotosensitive layer is subsequently developed, so that the irradiatedelements are dissolved and a series of depressions are formed in thephotosensitive layer. Next, a comparatively thin aluminum layer issputtered onto the photosensitive layer, which aluminum layer issubsequently provided with a comparatively thick nickel layer by meansof an electro deposition process. The nickel layer thus formed issubsequently removed from the substrate and forms the master mold to bemanufactured, which is provided, in the manner described above, with adisc-shaped surface having a series of raised portions corresponding tothe desired series of information elements on the information carrier tobe manufactured. The master mold thus manufactured can suitably be usedin the manufacture of the desired information carriers, however, ingeneral, a number of copies, so-called daughter molds are made by meansof the master mold in a replica process. These daughter moulds are usedto manufacture the desired information carriers by means of a furtherreplica process, generally an injection molding process. In this manner,the required number of master molds, which are comparatively expensive,is limited. Such a method of manufacturing an optically scannableinformation carrier, such as a CD or DVD, having pit-shaped informationelements by means of a master mold or by means of a daughter moldmanufactured by means of the master mold is commonly known andcustomary.

The interspace between the photosensitive layer and the lens of the lenssystem facing the photosensitive layer is filled with water. For thispurpose, the known device is provided with an outflow opening, which issituated near the axis of rotation of the table. The water supplied viathe outflow opening is spread, under the influence of centrifugalforces, substantially throughout the surface of the photosensitivelayer, so that also the interspace is filled with water. Since water hasa considerably larger optical refractive index than air, the provisionof water in the interspace leads to a substantial increase of an anglewhich the rays originating from the radiation beam and the optical axisof the lens system include at the location of the scanning spot. As aresult, the size of the spot formed by the radiation beam on thephotosensitive layer is reduced considerably, so that a much largernumber of irradiated and non-irradiated elements can be formed on thephotosensitive layer, and the information carrier to be manufactured hasa higher information density.

Another example of an application in which the gap between a lens and asurface to be irradiated is maintained filled with a liquid are opticalimaging methods and apparatus, such as optical projection lithograpy, inwhich the spot formed by the radiation projected onto the surface formsan image or a partial image. Such a method and apparatus are describedin international patent application WO99/49504.

A drawback of these methods and devices is that the liquid film formedin the interspace is not always reliably maintained fully and inhomogenous condition during and after relative displacement of the lensand the surface parallel to the surface. As a result, faults develop inthe photosensitive layer. In addition, variations in the condition ofthe liquid film caused by relative movements of the lens and the surfaceresult in varying forces being exerted on the lens system. Since thelens system is suspended with a limited rigidity, the varying forcesexerted by the liquid film cause undesirable vibrations of the lenssystem, which further disturb the precision with which the image isprojected onto the surface. Furthermore, a comparatively large quantityof liquid must be supplied to keep a liquid volume in place in theportion of the interspace through which the radiation passes. As aresult, the known device must be provided with extensive measures toprevent undesirable contact between the liquid and other parts of thedevice.

It is an object of this invention to reliably maintain the portion ofthe interspace between the optical surface nearest to the layer to beirradiated and that layer through which the radiation passes filled withliquid throughout a larger range of relative velocities and directionsof relative displacement of the optical element and the layer.

It is another object of the invention to reduce the risk of damage dueto unintentional contact between the optical element and the layer to beirradiated.

The canal distributing supplied liquid longitudinally along the canaland dispensing distributed liquid towards the layer feeds the liquid asa layer. Accordingly, the portion of the interspace fulfilling anoptical function can be maintained filled with liquid with lesssensitivity to variations in direction and velocity of the movement ofthe lens or lenses and the layer relative to each other.

That the method and the device are less sensitive to the velocity anddirection of displacement of the optical element and the layer relativeto each other and variations therein, is not only advantageous in themanufacturing of optical information carriers or molds therefor in whichthere are at most quite small variations in the direction of movement ofthe layer relative to the optical element, but also in otherapplications, such as optical imaging, and more in particular in forinstance wafer steppers and wafer scanners for optical projectionlithography for example for the production of semiconductor devices inwhich the direction of movement of the optical element relative to thelayer is varied substantially when the wafer is stepped relative to theoptical element to bring the optical element into a new positionopposite the wafer for projecting the reticle onto a new spot on thewafer or for unrolling (scanning) the projected image of the reticle(mask) over a next area on the wafer. The spot is then formed either bythe area of projection of the reticle onto the wafer or by the movingarea of projection of a running, usually slit shaped, window portion ofthe reticle obtained by or as if scanning along the reticle inaccordance with movement of the wafer relative to the optical element.

Other objects, features and effects as well as details of this inventionappear from the detailed description of a preferred form of theinvention.

FIG. 1 is a schematic side view of an example of a device for directingradiation to a spot on a layer;

FIG. 2 is a schematic, cross-sectional view of a distal end portion of afirst example of an optical system for a device as shown in FIG. 1, of alayer to which the radiation is directed and of a liquid flow maintainedin operation;

FIG. 3 is a schematic, bottom view along the line III-III in FIG. 2

FIG. 4 is a schematic, cross-sectional view of a distal end portion of athird example of an optical system for a device as shown in FIG. 1, of alayer to which the radiation is directed and of a liquid flow maintainedin operation;

FIG. 5 is a schematic, bottom view along the line V-V in FIG. 4; and

FIG. 6 is a schematic top plan view representation of a waferstepper/scanner for optical lithography.

In the manufacture of an optically scannable information carrier, suchas a CD or a DVD, a disc-shaped substrate 3 of glass (see FIG. 1)carrying a thin photosensitive layer 5 on one of its two sides isirradiated by means of a modulated radiation beam 7, for instance a DUVlaser beam with a wavelength of approximately 260 nm. To irradiate thephotosensitive layer 5, use is made of an example 25 of a device inaccordance with the invention, which device is described hereinafterwith reference to FIGS. 1-3. The radiation beam 7 is focused to ascanning spot 11 on the photosensitive layer 5 by an optical system,according to the present example in the form of a lens system 9,including a plurality of optical elements in the form of lenses. Thelens system 9 includes an objective lens 55, which is secured in a lensholder 57. The lens system 9 further includes a most distal lens 59,which is the one of the optical elements of the lens system 9 that islocated nearest to the layer 5 when in operation. An interspace 53 ismaintained between the layer 5 that is irradiated and the one of theoptical elements of the lens system 9 that is located nearest to thelayer 5. The optical elements may also include other items than lenses,such as filters, shields, diffraction gratings or mirrors.

The layer 5 and the lens system 9 are displaced with respect to eachother, so that the modulated radiation beam 7 on the photosensitivelayer 5 successively irradiates a series of spaced apart irradiatedportions of the layer 5 and does not irradiate portions of the layer 5in-between the irradiated portions. The irradiated photosensitive layer5 is subsequently developed by means of a developing liquid, whichdissolves the irradiated elements 13 and leaves the non-irradiatedelements 15 on the substrate 3. It is also possible to provide that theirradiated portions are left while the non-irradiated portions aredissolved. In both cases, a series of pits or bumps, which correspondsto the desired series of pit-shaped information elements on theinformation carrier, are formed in the photosensitive layer 5. Thephotosensitive layer 5 is subsequently covered with a comparatively thinlayer of for instance nickel by means of a sputtering process.Subsequently, this thin layer is covered with a comparatively thicknickel layer in an electro deposition process. In the nickel layer,which is eventually removed from the substrate 3, the pattern of pitsformed in the photosensitive layer 5 leaves a corresponding pattern thatis a negative of the pattern to be formed in the information carrier tobe manufactured, i.e. the master mold comprises a series of raisedportions, which correspond to the series of pit-shaped elements formedin the photosensitive layer 5 and to the desired series of pit-shapedinformation elements on the information carrier. The master mold is thusrendered suitable for use as a mold in an injection-molding machine forinjection molding the desired information carriers. Generally, however,a copy of the master mold is used as the mold for injection moldinginstead of the master mold, which copy of the master mold is commonlyreferred to as daughter mold, which is manufactured by means of themaster mold using a customary replica process which is known per se.

The substrate 3 with the photosensitive layer 5 is placed on a table 27that is rotatable about an axis of rotation 29, which extendsperpendicularly to the table 27 and the substrate 3. The table can bedriven by means of a first electromotor 31. The device 25 furthercomprises a radiation source 33, which, in the example shown, is a lasersource, which is secured in a fixed position to a frame 35 of the device25. It is observed that, as an alternative, the radiation may also beobtained from outside the device. Control over the radiation directed tothe layer 5 can be achieved in many ways, for instance by controllingthe radiation source 33 and/or by controlling a shutter or radiationdiverter (not shown) between the radiation source 33 and the layer 5.

The optical lens system 9 is secured onto a first traveller 37, whichcan be displaced radially (parallel to the X-direction in the drawings)relative to the axis of rotation 29, by means of a first displacementstructure 39. For this purpose, the first displacement structure 39includes a second electromotor 41 by means of which the first traveller37 can be displaced over a straight guide 43, which extends parallel tothe X-direction and is fixed relative to the frame 35.

A mirror 45 in line with an optical axis 49 of the lens system 9 is alsosecured to the first traveller 37. In operation, the radiation beam 7generated by the radiation source 33 follows a radiation beam path 47extending parallel to the X-direction, and the radiation beam 7 isdeflected by the mirror 45 in a direction parallel to the optical axis49 of the lens system 9. The lens system 9 can be displaced in thedirection of its optical axis 49 by means of a focus actuator 51, overcomparatively small distances with respect to the first traveller 37, sothat the radiation beam 7 can be focused on the photosensitive layer 5.The table 27 with the substrate 5 is rotated about the axis of rotation29 at a comparatively high speed by means of the first motor 31, and thelens system 9 is displaced parallel to the X-direction by means of thesecond motor 41 at a comparatively low speed, so that the scanning spot11 where the radiation beam 7 hits the layer follows a spiral-shapedtrack over the photosensitive layer 5, leaving a trail of irradiated andnon-irradiated elements extending in accordance with this spiral-shapedtrack.

The device 25 can suitably be used to manufacture master molds having acomparatively high information density, i.e. by means of the device 25,a comparatively large number of irradiated elements can be provided perunit area of the photosensitive layer 5. The attainable informationdensity increases as the scanning spot 11 is smaller. The size of thescanning spot 11 is determined by the wavelength of the radiation beam 7and by the numerical aperture of the lens system 9, the numericalaperture depending upon the optical refractive index of the mediumpresent between the lens system 9 and the photosensitive layer 5. Thescanning spot 11 is smaller as the refractive index of the mediumpresent between the lens system 9 and the photosensitive layer 5 islarger. Liquids typically have a much larger optical refractive indexthan air and therefore the portion of the interspace 53 between the lenssystem 9 and the photosensitive layer 5 through which the beam 7 extendsis maintained filled with a liquid—according to this example water. Inthe present example, water is also particularly suitable because it istransparent to the DUV radiation beam 7 used and it does not attack thephotosensitive layer 5.

As shown in FIG. 1, the device 25 according to the present examplefurther includes a liquid removal structure 77, which is provided with apick-up mouth 79. The pick-up mouth 79 is secured onto a secondtraveller 81 of the device 25, which can be displaced by means of asecond displacement structure 83 of the device 25 in a radial directionwith respect to the axis of rotation 29, according to the presentexample parallel to the X-direction, but another radial direction ofdisplacement may be provided. For driving the displacement of the secondtraveller 81, the second displacement device 83 comprises a thirdelectromotor 85 connected to the second traveller 81 for displacing thesecond traveller over a straight guide 87, which is attached to theframe 35 and extends in the directions of displacement of the secondtraveller 81.

In operation, the pick-up mouth 79 is displaced by means of the thirdmotor 85. The third motor 85 is controlled so that the lens system 9 andthe pick-up mouth 79 are continuously situated at substantially equaldistances R from the axis of rotation 29 of the substrate 3. In thismanner, the pick-up mouth 79 is maintained in a position downstream fromthe lens system 9 where irradiated portions of the layer 5 pass, so thatthe liquid supplied at the location of the lens system 9 is entrained bythe rotating layer 5 to the pick-up mouth 79 where the liquid issubsequently picked-up from the photosensitive layer 5 by the pick-upmouth 79. As the water is thus removed from the photosensitive layer 5downstream from the lens system 9, it is substantially precluded thatwater that has already been used finds its way back to the interspace53, thereby disturbing the accurately dosed liquid flow in theinterspace 53. In operation, the pick-up mouth 79 is always at adistance R from the axis of rotation 29 which corresponds to thedistance R at which the lens system 9 is situated from the axis ofrotation 29, both the size and the capacity of the pick-up mouth 79 needonly to be comparatively small to remove liquid that has already beenused.

FIGS. 2 and 3 show, in more detail, the lens system 9, the substrate 3with the photosensitive layer 5, and the interspace 53 between thephotosensitive layer 5 and the lens system 9. The lens 59 nearest to thelayer 5 has an optical surface 63 facing the substrate 3 and nearest tothe substrate 3. The lenses 55, 59 are suspended in a housing 61, whichincludes a flat wall 65, which faces the layer 5 and which substantiallyextends in an imaginary plane perpendicular to the optical axis of thelens 59 nearest to the layer 5. In the surface of the lens systemnearest to the layer 5, a recess 92 facing the spot 11 to which theradiation 7 is directed is provided. The surface 63 of the lens 59nearest to the layer 5 forms an internal surface of the recess 92. Thissurface 63 also bounds the portion of the interspace 53 through whichthe radiation 7 irradiates the spot 11. According to the presentexample, the surface 63 of the lens 59 nearest to the layer 5 is concaveso that the deepest point of the recess 92 is in the middle, however,this surface may also be flat or convex.

In operation, the portion of the interspace 53 through which theradiation 7 irradiates the spot 11 on the layer 5 is maintained filledwith liquid 91. In the recess 92, the liquid 91 is, at least to someextent, protected against being entrained from the interspace 53. Since,the liquid 91 is less susceptible to being entrained away from theportion of the interspace 53 through which the radiation passes to thespot 11, occurrence of the associated optical distortion caused by theportion of the interspace 53 through which the radiation passes notbeing completely filled with liquid is thus counteracted.

Moreover, this allows the smallest size of the interspace 53 measuredparallel to the optical axis of the lenses 55, 59 to be relativelylarge. In turn, this reduces the risk of damage to the lens 59 nearestto the layer 5 and the allowable tolerances on the tilt of the lens canbe larger without increasing the risk of the lens 59 touching the layer5.

The recess 92 may for instance be positioned and of such dimensions, sothat only a portion of the radiation passes through the recess. However,for a particularly effective protection of liquid 91 across the wholeradiation beam, it is preferred that the recess 92 has a rim portion 94closest to the layer 5, which extends around the radiation 7 irradiatingthe spot 11. Accordingly, the portion of the interspace 53 in the recess92 in which liquid 91 is shielded from being entrained extendsthroughout the whole cross-section of the radiation beam.

The optimum working distance between the layer 5 and the wall 65, i.e.the portion of the lens assembly nearest to the layer 5, is determinedby two factors. On the one hand, the distance should be large enough toretain sufficient tolerance on the distance between the substrate 3 andarrangement of the lenses 55, 59 and the housing 61. On the other hand,this distance should not be too large because this would require a toolarge liquid flow to maintain the immersed condition of the portion ofthe interspace 53 through which the radiation passes to the spot 11. Apresently preferred range for the smallest thickness of the interspace53 is 3-1500 μm and more preferably 3-500 μm if the liquid is waterlarger values for the smallest thickness of the interspace can beparticularly advantageous if the liquid has a larger viscosity thanwater. Also the overall width of the outflow opening affects the upperend of the preferred range for the smallest thickness of the interspace,the smallest thickness of the interspace being preferably smaller than(100+ 1/20*W)μm in which W is the overall width of the outflow openingmeasured in a plane parallel to the layer 5.

The smallest thickness of the interspace may be larger thanapproximately 10 μm, for instance larger than 15 μm, 30 μm or even 100μm, to increase the insensitivity to tolerances.

To avoid inclusion of air bubbles in the liquid and for reliablymaintaining the filled condition of the portion of the interspace 53through which the radiation 7 passes to the spot 11, liquid outflow ispreferably such that a liquid volume between the wall 65 and the layer 5is maintained which includes a portion of the interspace 53 upstream (ina direction opposite to the direction of relative movement of the layer5 in the area of the spot 11) of the portion of the interspace 53through which the radiation irradiates the spot 11. Thus, a safetymargin of liquid upstream is formed which ensures that variations in thedistance over which liquid is urged in upstream direction do not cause adisruption of the filled condition of the portion of the interspace 53through which the radiation 7 passes to the spot 11.

The most downstream outflow opening 90 in the lens system 9 throughwhich the liquid 91 is passed has a total projected cross-sectionalpassage area in a plane parallel to the layer 5 of which, seen in adirection parallel to the optical axis of the lens system 109, thecentre is located inside the portion of the interspace 53 through whichthe radiation 7 irradiates the spot 11. Accordingly, the average pathalong which liquid flows out is at least to a large extent centredrelative to the portion of the interspace 53 through which radiationpasses to the spot 11. Accordingly, the direction of movement of thelayer 5 and the lens arrangement 9 relative to each other in the area ofthe spot 11 can be varied substantially without disrupting completeimmersion of the portion of the interspace 53 through which the spot 11is irradiated. Even if the direction of movement of the layer 5 isvaried substantially, the trace of liquid 95 will still cover the entireportion of the interspace 53 through which the spot is irradiated.Nevertheless, areas of the outflow opening 90 around the beam 7 arelocated close to the beam, so that superfluous wetting of the layer 5 islimited.

According to the present example, the portion of the interspace 53through which the radiation 7 irradiates the spot 11 is also centrallylocated relative to the outflow opening 90 to such an extent that thetrace 95 of liquid 91 fed from the outflow opening 90 into theinterspace 53 completely immerses the portion of the interspace 53through which the radiation 7 irradiates the spot 11, not only while, inthe position of the spot 11, the layer 5 and the at least one lenssystem 9 move relative to each other in the direction indicated by thearrow 52 (which indicates the direction of movement of the layer 5relative to the lens system 9), but also while, in the position of thespot 11, the layer 5 and lens system 9 move relative to each other inopposite direction.

The more the direction of movement of the layer 5 and the lens system 9parallel to the layer 5 in the area of the spot 11 can be changedwithout disrupting the immersion of the portion 194 of the area 153through which the radiation passes, the more the device is suitable forapplications in which the spot 11 needs to move over the surface of thelayer in widely varying directions, such as in imaging processes inwhich the spot is a two-dimensional image projected to the layer 5. Insuch applications, the advantage of a comparatively large refractiveindex between the lens system and the medium between the lens system andthe irradiated surface is that the image can be projected with a higherresolution, which in turn allows further miniaturization and/or animproved reliability.

An example of such applications is optical projection lithography forthe processing of wafers for the manufacture of semiconductor devices.An apparatus and a method for this purpose are schematically illustratedin FIG. 6. Wafer steppers and wafer scanners are commercially available.Accordingly, such methods and apparatus are not described in greatdetail, but primarily to provide an understanding of liquid immersion asproposed in the present application in the context of such opticalimaging applications.

The projection lithography apparatus according to FIG. 6 includes awafer support 12 and a projector 113 having a lens assembly 14 above thewafer support 12. In FIG. 6, the wafer support 12 carries a wafer 15 onwhich a plurality of areas 16 are intended to be irradiated by a beamprojecting an image or partial image of a mask or reticle 17 in ascanner 18 operatively connected to the projector 113. The support tableis moveable in X and Y direction along spindles 19, 20 driven by spindledrives 21, 22. The spindle drives 21, 22 and the scanner 18 areconnected to a control unit 23.

Usually one of two principles of operation are applied in opticallithography. In the so-called wafer stepper mode, the projector projectsa complete image of the reticle onto one of the areas 16 on the wafer15. When the required exposure time has been reached, the light beam isswitched off or obscured and the wafer 15 is moved by the spindle drives21, 22 until a next area 16 of the wafer is in the required position infront of the lens assembly 14. Dependent on the relative positions ofthe exposed area and the next area to be exposed, this may involverelatively quick movement of the lens assembly 14 along the surface ofthe wafer in widely varying directions. The size of the irradiated spoton the surface of the wafer in which the image of the reticle isprojected is typically about 20×20 mm, but larger and smaller spots areconceivable.

In particular when it is desired to manufacture larger semiconductorunits, it is advantageous to project the image in the other mode,usually referred to as the wafer scanner mode. In that mode, only aslit-shaped portion of the reticle is projected as a slit shaped spothaving a length that is several (for instance four or more times) timeslarger than its width in an area 16 on the surface of the wafer 15. Atypical size for the spot is for instance 30×5 mm). Then, the reticle 17to be scanned is moved along the scanning window while the wafer support12 is synchronously moved relative to the lens assembly 14 under controlof the control unit 23 with a velocity adapted so that only theprojection spot, but not the scanned partial image portions of thereticle 17 that are projected on the wafer move relative to the wafer15. Thus, the image of the reticle 17 is transferred to an area 16 ofthe wafer as successive portions “unroll” as the spot progresses overthe wafer. The movement of the wafer 15 relative to the lens assembly 14while a running window portion of the reticle is projected onto thewafer 15 is usually carried out relatively slowly and usually each timein the same direction. After the complete image of a reticle 17 has beenprojected onto the wafer 15, the wafer 15 is generally moved much morequickly relative to the lens assembly 14 to bring a next area of thewafer 15 where a next image of the or a reticle 17 is to be projected infront of the lens assembly 14. This movement is carried out in widelyvarying directions dependent on the relative positions of the exposedarea 16 of the wafer 15 and the next area 16 of the wafer 15 to beexposed. To be able to recommence irradiating the surface of the wafer15 after the displacement of the wafer 15 relative to the lens 14 (i.e.also the lens or the lens and the wafer may be moved), it isadvantageous if the liquid volume in the interspace between the lens 14and the surface of the wafer 15 through which the radiation passes isimmediately filled with liquid after completion of that movement, sothat the space is reliably immersed before radiation is recommenced.

Also for optical lithography, water can be used, for instance if theradiation is light of a wavelength of 193 nm. However in somecircumstances other liquids may be more suitable.

Returning to FIGS. 2 and 3, since the recess 92 is bound by a concaveportion of the surface 63 of the lens 59 nearest to the spot 11 on thelayer 5 to which the beam of radiation 7 is directed, the advantages ofhaving a recess 92 are combined with a relatively uniform flow patternthroughout the portion 94 of the interspace 53 through which radiation 7passes to the spot 11. In particular, a uniform pattern of flow velocitygradients in the interspace 53 is obtained. In turn, the relativelyuniform flow pattern is advantageous to avoid inducing vibrations andfor obtaining a continuous uniform supply of fresh liquid and thereby auniform, steady liquid temperature. These effects are both advantageousfor avoiding optical disturbance of the radiation beam 7.

In FIG. 3, the circle designated by reference numeral 94 indicates theperimeter of the portion of the interspace 53 between the lens 59 andthe layer 5 through which the radiation beam 7 passes.

For supplying liquid 91 to the interspace 53 between the lens 59 and thelayer 5, a liquid supply conduit 67 extends through the housing 61 andleads to an outflow opening 90. According to the present example, theoutflow opening 90 has the form of a canal structure in a surface 54,which canal structure 67 is open towards the layer 5, for distributingsupplied liquid 91 longitudinally along the canal 67 and dispensingdistributed liquid towards the layer 5. In operation, the liquid 91 isdistributed by the canal structure 67 longitudinally along that canalstructure and the liquid 91 is dispensed from the canal structure 67towards the layer 5. This results in a relatively wide liquid trace 95and full immersion of the portion 94 of the interspace 53 through whichthe radiation beam 7 passes, even if the direction of movement of thelens system 9 and the layer 5 relative to each other parallel to theplane of the layer 5 is changed substantially.

The canal 67 can have various forms. In the embodiment shown in FIGS. 2and 3, the canal is formed such that the outflow opening 90 is locatedoutside the radiation beam 7 and extends around the portion 94 of theinterspace 53 through which the radiation 7 irradiates the spot 11. Thecross 96 indicates the centre, seen in a direction parallel to theoptical axis of the lens system 9, of the total cross-sectional passagearea of the outflow opening 90.

The liquid 91 is preferably supplied at a pressure drop over the liquidbetween the canal structure 67 and the environment that is justsufficient to keep portion of the interspace 53 through which theradiation passes reliably immersed. Thus, the amount of water fed to thesurface is kept to a minimum.

Furthermore, when the liquid 91 is dispensed via a canal shaped outflowopening 90, the smallest thickness of the interspace 153 (in thisexample the distance between the layer 5 and the surface 54 of the wallportion 65) may be larger, without causing an undue risk of disruptingthe immersion of the portion 94 of the interspace through which theradiation passes. Therefore, when the liquid is dispensed from acanal-shaped outflow opening 90, the displacement structure 39, 83 andthe lens system 9 are preferably positioned and dimensioned formaintaining the smallest thickness of the interspace 53 in a rangebetween 3 and 500 μm.

The flow rate with which the liquid 91 is supplied is preferably suchthat it can be reliably ensured that a laminar flow with an essentiallylinear velocity profile and preferably a homogeneous Couette flow ispresent in the interspace 53. Such a flow exerts a substantiallyconstant force on the wall 65 in which the canal 67 is provided and onthe side 63 of the lens 59 nearest to the layer 5. As a result, theliquid present in the interspace 53 exerts substantially no variableliquid forces on the lens system 9. Such varying liquid forces wouldlead to undesirable vibrations of the lens system 9 and hence tofocusing errors and positioning errors of the radiation beam 7 on thephotosensitive layer 5. The flow is preferably free of air inclusions,so that the radiation beam 7 is not disturbed.

In FIGS. 4 and 5 a second example of a lens system 109 for devices suchas the devices shown in FIGS. 1 and 6 is shown. According to thisexample, the outflow opening 190 downstream of the liquid supply canal167 is also provided with a canal structure open towards the layer 5(i.e. in the direction in which the beam 107 is directed), but has adifferent, rectangular shape when seen in axial direction of the lenssystem 109. An essentially rectangular shape is particularlyadvantageous for reliably immersing a rectangular area 194 of theinterspace intersected by the radiation beam while maintaining a uniformliquid flow patter throughout the intersected portion 194 of theinterspace 153, in particular if the movement of the lens system 109 andthe layer 5 relative to each other is in a direction perpendicular toone of the sides of the rectangular canal structure 190. Suchcircumstances typically occur in optical projection lithography.

The recess 192 is bounded by a passage 195 in a wall 165 perpendicularto the axis of the lens system 9 and a surface of the lens 159 nearestto the spot 11 and the surface of the lens 159 nearest to the spot 11also bounds the portion 194 of the interspace 153 through which theradiation 107 passes to the spot 11. Accordingly, the lens 159 iseffectively protected against damage due to inadvertent contact betweenthe lens system 109 and the layer 5 on the substrate 3.

1. A method of irradiating a layer including: directing and focussing aradiation beam to a spot on said layer by means of at least one opticalelement; causing relative movement of the layer relative to said atleast one optical element so that, successively, different portions ofthe layer are irradiated and an interspace between a surface of said atleast one optical element nearest to said layer is maintained; andmaintaining at least a portion of said interspace through which saidradiation beam irradiates said spot on said layer filled with a liquid,the liquid being supplied via a supply conduit and flowing out of anoutflow opening; wherein at least one outflow opening via which theliquid flows out is provided in the form of at least one canal opentowards said layer, said canal distributing supplied liquidlongitudinally along said canal and dispensing distributed liquidtowards said layer, and wherein at least a portion of said liquid fillsup a recess through which said radiation beam irradiates said spot, saidrecess including a concave portion of said surface of said at least oneoptical element nearest to said layer.
 2. The method according to claim1, wherein the canal or canals are positioned such that, seen in adirection perpendicular to said layer, the canals define a totalcross-sectional area having a centre in said portion of said interspacethrough which the radiation irradiates the spot.
 3. The method accordingto claim 1, wherein a smallest thickness of said interspace ismaintained at 3-1500 μm.
 4. The method according to claim 1, wherein therecess has a rim portion closest to said layer extending around saidradiation irradiating said spot.
 5. A device for directing radiation toa layer including: at least one optical element for focussing a beam ofradiation originating from a radiation source to a spot on said layer; adisplacement structure for causing relative movement of the layerrelative to said at least one optical element so that, successively,different portions of the layer are irradiated and an interspace betweensaid layer and a surface of said at least one optical element nearest tosaid spot is maintained; and an outflow opening for supplying liquid toat least a portion of said interspace through which, in operation, saidradiation irradiates said spot on said layer, said outflow openinghaving a total projected cross-sectional passage area in a planeperpendicular to an axis of said beam; wherein the outflow opening isformed by at least one canal open towards said layer, for distributingsupplied liquid longitudinally along said canal and dispensingdistributed liquid towards said layer, and wherein a recess is providedin a surface facing said spot, an internal surface of said recessbounding at least said portion of said interspace through which saidradiation irradiates said spot, said recess including a concave portionof said surface of said at least one optical element nearest to saidspot.
 6. The device according to claim 5, wherein the outflow opening ora plurality of the outflow openings are positioned such that, seen in adirection parallel to said axis of said beam, said total projectedcross-sectional passage area has a centre in said portion of saidinterspace through which the radiation irradiates the spot.
 7. Thedevice according to claim 5, wherein said displacement structure andsaid recess are positioned and dimensioned for maintaining a smallestthickness of said interspace at 3-1500 μm.
 8. The device according toclaim 5, wherein said recess has a rim portion closest to said layerextending around said portion of said interspace through which, inoperation, said radiation irradiates said spot.